1. Field of the Invention
The present invention relates generally to an optical fiber cable for use mainly in indoor wiring, and particularly to an optical fiber cable provided with at least one anti-shrink member and a plastic jacket.
2. Description of the Background Art
An optical fiber cable for use in indoor wiring usually has a structure in which tension members are placed around a plurality of optical fibers, and a plastic jacket is provided as an outer covering. It is usual that the jacket shrinks longitudinally with time due to residual stresses produced during the covering process. When the longitudinal shrinkage of the jacket is large, stresses are applied to the optical fibers. The stresses may affect the transmission properties of the optical fibers.
To suppress the shrinkage of the jacket, researchers and engineers have been studying a structure in which anti-shrink members are embedded in the jacket. On the other hand, as multi-fiber connectors are widely used for connecting a plurality of optical fibers as one unit, optical transmission lines frequently use optical fiber cables having a fiber ribbon, in which multiple optical fibers are organized in a flat array. Japanese patent 2793621 has disclosed a structure in which a fiber ribbon is enclosed by a jacket in which tension members are embedded. The tension members also function as anti-shrink members for suppressing the longitudinal shrinkage of the jacket.
An object of the present invention is to solve the above-described problem and to offer an optical fiber cable having excellent workability and long-term reliability.
According to the present invention, the foregoing and other objects and advantages are attained by offering an optical fiber cable described below. The optical fiber cable comprises the following components:
(a) at least one optical fiber;
(b) a plastic jacket covering the optical fiber or optical fibers; and
(c) at least one anti-shrink member that is embedded in the jacket and that is intended to suppress the longitudinal shrink of the jacket.
In this cable, the jacket has the property that the longitudinal shrinkage is at most 0.5% when a sample of the jacket including the anti-shrink member or members but excluding the optical fiber or fibers is heated at 110xc2x0 C. for two hours. The optical fiber cable has the following properties:
(a) The remaining bend of the cable has a radius of curvature of at least 100 mm when a sample of the cable is wound on a mandrel having a radius of 50 mm, is secured there, is heated at 85xc2x0 C. for two hours, and then is unwound from the mandrel.
(b) The deflection at the free end of a cantilever is at least 50 mm when the cantilever is formed by using a sample of the cable having a length of 30 cm and when the deflection is obtained by averaging the maximum deflection and the minimum deflection. In this case, the value of the maximum deflection is obtained by turning the cable sample around its own axis to find the position where the deflection at the free end is maximized due to the bending tendency of the cable sample. The value of the minimum deflection is obtained by turning the cable sample again around its own axis to find the position where the deflection at the free end is minimized due to the bending tendency.
In accordance with one aspect of the invention, the optical fiber cable has the following properties:
(a) The ratio ESt/ESj is at least 0.7, where ESt denotes the product of the Young""s modulus and cross-sectional area of the anti-shrink member or the total value of the products of the Young""s modulus and cross-sectional area of the anti-shrink members, and ESj denotes the product of the Young""s modulus and cross-sectional area of the entire jacket including the anti-shrink member or members;
(b) The ratio EIt/EIc is at least 0.1, where EIt denotes the flexural rigidity of the anti-shrink member or the total value of the flexural rigidities of the anti-shrink members (the flexural rigidity is expressed by the product of the Young""s modulus and the geometrical moment of inertia), and EIc denotes the flexural rigidity of the entire cable.
(c) The ratio EIc/Mc is at most 8xc3x97106 mm3, where EIc denotes the flexural rigidity of the entire cable, and Mc denotes the mass per unit length of the cable.
The optical fiber cable may have the dimensional relationship expressed as
Ttxe2x89xa6To+0.2 (mm), 
where Tt is the total thickness of the jacket at the portion where the anti-shrink member or members are embedded, and To is the thickness of the jacket at the portion where no anti-shrink member is embedded.
In the optical fiber cable, the anti-shrink member or members may be coated with a bonding material by baking. In this case, the bonding material may have a thickness of at most 50 xcexcm. The bonding material before being applied onto the anti-shrink member or members may be composed of an acrylic-resin-family bonding material dispersed in a solvent.
The optical fiber cable may further comprise a tension member surrounding the optical fiber or fibers and being surrounded by the jacket.
In the optical fiber cable, the jacket may be made of polyvinyl chloride (PVC), and the anti-shrink member or members may be made of glass-fiber-reinforced plastic (G-FRP).
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.